Craters What are they good
for?
Density of cratering
is the principal method by which we can date the surfaces of most
planets/satellites.
Impact flux (impacts per time
on a surface) has not been constant throughout the history of the solar system.
The oldest lunar surfaces (4.6 to
≈ 3.9 billion years ago) literally saturated with impact craters. It is estimated that the frequency of impacts
was at least 100 times higher than present impact flux during this early part
of the Solar System.
Resurfacing of a body destroys evidence of earlier cratering.
The density of craters on a surface can therefore be used to give an estimate
of the age of that surface.
Crater Morphology (appearance)
There is a correlation between crater size and appearance.
Small craters simple bowl shape
Larger craters terracing of the rim and central peaks result from the slumping
of the inner walls and rebounding of the depressed crater floor.
Largest craters - the single central peak is replaced by one or more peak
rings, resulting in what are generally termed impact basins.
Complicating Factors
e.g. Earth - higher gravity on Earth results in lower diameter range
for each morphological type on Earth.
Craters on the
Earth
Earths system of Plate Tectonics resurfaces much the planet, eliminating
much evidence of craters. Nevertheless more than 150 craters have been
identified.
Some noteworthy examples:
Cicxulub impact 64.98 million yrs
ago, 170 km diameter, mass extinction of dinosaurs
Zhamanshin in Kazaksthan
- 1 million years, 15 km diameter, most recent large impact
tens of metres, body at an altitude of 10 km